This invention relates to a process for making absorbent material useful in personal care absorbent articles, medical absorbent articles and the like, in which a superabsorbent polymer component of the absorbent material is synthesized during manufacture of the absorbent material.
Processes for forming absorbent composite materials from cellulose fibers and the like are known. U.S. Pat. No. 5,350,624 to Georger et al.; U.S. Pat. No. 4,902,559 to Eschwey et al.; U.S. Pat. No. 4,818,464 to Lau and U.S. Pat. No. 4,100,324 to Anderson et al. disclose processes for combining absorbent cellulose fibers with thermoplastic fibers to make absorbent composites. At least one meltblown die head, used for making meltblown fibers, is arranged near a chute, or as in the case of Eschwey the continuous fibers are spun into a duct containing fiberized pulp. Cellulose fibers and, possibly, other materials are injected from the chute into the nascent fiber stream while the fibers are forming. Pre-formed particles or fibers of superabsorbent material may also be added through the chute. These processes are generally referred to as xe2x80x9ccoformxe2x80x9d processes.
Various processes for making superabsorbent polymers which are useful in absorbent composite materials are also known. U.S. Pat. No. 5,962,068, issued to Tsuchiya et al., discloses a process for producing a water-absorptive composite. First, an aqueous monomer solution containing a polymerizable monomer capable of producing a water-absorptive polymer is provided. Then, polymerization is initiated using a redox polymerization initiator. The resultant reaction mixture, which is partially polymerized, is applied dropwise onto a fibrous substrate. The polymerization is completed on the substrate.
One feature that the known processes have in common is that they require at least some separate process steps for polymerizing or partially polymerizing the superabsorbent material before it can be added to the forming process for the absorbent composite. In other words, neither process totally integrates the superabsorbent polymer formation with the ultimate process for forming the absorbent composite.
The term xe2x80x9ccellulose fibersxe2x80x9d refers to fibers from natural sources such as woody and non-woody plants, regenerated cellulose, and derivatives from these fibers by means of chemical, mechanical or thermal treatment, or any combination of these. Woody plants include, for example, deciduous and coniferous trees. Non-woody plants include, for instance, cotton, flax, esparto grass, milkweed, straw, jute hemp, and bagasse. Regenerated cellulose fibers include, for instance, viscose and rayon. The cellulose derivatives include, for instance, microcrystalline cellulose, chemically crosslinked fibers, and chemically uncrosslinked, twisted fibers.
The term xe2x80x9caverage pulp fiber lengthxe2x80x9d refers to a weighted average length of pulp determined using a Kajaani fiber analyzer Model No. FS-100 available from Kajaani Oy Electronics in Kajaani, Finland. Under the test procedure, a fiber sample is treated with a macerating liquid to ensure that no fiber bundles or shives are present. Each fiber sample is dispersed in hot water and diluted to about a 0.001% concentration. Individual test samples are drawn in approximately 50 to 500 ml portions from the dilute solution and tested using the standard Kajaani fiber analysis procedure. The weighted average fiber lengths may be expressed by the following equation:       ∑                  X        1             greater than       0        k    ⁢      xe2x80x83    ⁢            (                        X          1                *                  n          i                    )        /    n  
where
k=maximum fiber length,
Xi=individual fiber length,
ni=number of fibers having length Xi 
and n=total number of fibers measured.
The term xe2x80x9cmeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity heated gas (e.g., air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed for example, in U.S. Pat. No. 3,849,241 to Butin et al. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in diameter, and are generally self bonding when deposited onto a collecting surface.
The term xe2x80x9cspunbonded fibersxe2x80x9d refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine capillaries of a spinnerette having a circular or other configuration, with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartman, U.S. Pat. No. 3,502,538 to Petersen, and U.S. Pat. No. 3,542,615 to Dobo et al. Spunbond fibers are quenched and generally not tacky on the surface when they enter the draw unit, or when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and may have average diameters larger than 7 microns, often between about 10 and 30 microns. In both cases above the fibers are attenuated to their final diameter by aerodynamic drawing processes.
The term xe2x80x9cstaple filaments or fibersxe2x80x9d means filaments or fibers which are natural or which are cut from a manufactured filament prior to forming into a web, and which have a length ranging from about 0.1 -15 cm, more commonly about 0.2-7 cm.
The term xe2x80x9cmicrofibersxe2x80x9d means small diameter fibers having an average diameter not greater than about 75 microns, for example, having an average diameter of from about 0.05 micron to about 50 microns, or more particularly, having an average diameter of from about 0.1 micron to about 10 microns, or even more typically 0.5 micron to about 5 microns.
The term xe2x80x9csubstantially continuous filaments or fibersxe2x80x9d refers to filaments or fibers prepared by extrusion from a spinnerette, including without limitation spunbonded and meltblown fibers, which are not cut from their original length prior to being formed into a nonwoven web or fabric. Substantially continuous filaments or fibers may have lengths ranging from greater than about 15 cm to more than one meter; and up to the length of the nonwoven web or fabric being formed. The definition of xe2x80x9csubstantially continuous filaments or fibersxe2x80x9d includes those which are not cut prior to being formed into a nonwoven web or fabric, but which are later cut when the nonwoven web or fabric is cut.
The term xe2x80x9cnonwoven fabric or webxe2x80x9d means a web having a structure of individual fibers or filaments which are interlaid, but not in an identifiable manner as in a knitted fabric. The terms xe2x80x9cfiberxe2x80x9d and xe2x80x9cfilamentxe2x80x9d are used herein interchangeably. Nonwoven fabrics or webs have been formed from many processes such as, for example, meltblowing processes, spunbonding processes, air laying processes, and bonded carded web processes. The term also includes films that have been perforated or otherwise treated to allow air to pass through. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.)
The term xe2x80x9cpolymerxe2x80x9d generally includes but is not limited to, homopolymers, copolymers, including block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term xe2x80x9cpolymerxe2x80x9d shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic and atactic symmetries.
The term xe2x80x9cwettablexe2x80x9d and/or xe2x80x9chydrophilicxe2x80x9d is meant to refer to a fiber which exhibits a liquid such as water, synthetic urine, or a 0.9 weight percent aqueous saline solution, in air contact angle of less than 90xc2x0. The contact angle may be determined, for example, in accordance with ASTM D724-89.
The term xe2x80x9cthermoplasticxe2x80x9d is meant to describe a material that softens and flows when exposed to heat and which substantially returns to its original hardened condition when cooled to room temperature.
The term xe2x80x9csuperabsorbent polymer precursor compositionxe2x80x9d refers to any and all solutions which, when mixed, chemically reacts to form a superabsorbent polymer. Each solution may be comprised of any combination of oligomer(s), monomer(s), crosslinking reagent(s), neutralizing agent, or initiator(s). In instances when only a single solution is utilized all the desired components must be in said solution and the initiator(s) must require a later activation step (e.g. heating or irradiation). In instances when two or more solutions are utilized the initiator(s) is most often, but not limited to, a chemical redox pair. When a redox pair, comprised of an oxidizing radical generator and a reducing agent, is used as the initiator the oxidizing radical generator and reducing agent must be in separate solutions. The solution of oxidizing radical generator or reducing agent may also contain any combination of oligomer(s), monomer(s), crosslinking reagent(s), or neutralizing agent.
The terms xe2x80x9celasticxe2x80x9d and xe2x80x9celastomericxe2x80x9d are used interchangeably to mean a material that is generally capable of recovering its shape after deformation when the deforming force is removed. Specifically, as used herein, elastic or elastomeric is meant to be that property of any material which upon application of an elongating force, permits that material to be stretchable to a stretched length which is at least about 25 percent greater than its relaxed length, and that will cause the material to recover at least 40 percent of its elongation upon release of the stretching elongating force. A hypothetical example which would satisfy this definition of an elastomeric material would be a one (1) inch sample of a material which is elongatable to at least 1.25 inches and which, upon being elongated to 1.25 inches and released, will recover to a length of not more than 1.15 inches. Many elastic materials may be stretched by much more than 25 percent of their relaxed length, and many of these will recover to substantially their original relaxed length upon release of the stretching, elongating force.
The term xe2x80x9crecoverxe2x80x9d or xe2x80x9cretractxe2x80x9d relates to a contraction of a stretched material upon termination of an elongating force following stretching of the material by application of the elongating force.
The term xe2x80x9csuperabsorbent materialxe2x80x9d refers to a water swellable, water-insoluble organic or inorganic material capable, under the most favorable conditions, of absorbing at least about 15 times its weight, preferably at least about 20 times its weight in an aqueous solution containing 0.9% by weight sodium chloride. The term xe2x80x9cabsorbent materialxe2x80x9d refers to any material capable of absorbing from about 5 to less than about 15 times its weight of the same solution.
The term xe2x80x9cpersonal care absorbent articlexe2x80x9d includes diapers, training pants, swim wear, absorbent underpants, adult incontinence products, feminine hygiene products, and the like.
The term xe2x80x9cmedical absorbent articlexe2x80x9d includes medical absorbent garments, drapes, gowns, bandages, wound dressings, underpads, wipes, and the like.
The term xe2x80x9ctissue and towel articlexe2x80x9d includes facial and bathroom tissues, paper towels, wet wipes, and the like.
The present invention is directed to a process for making an absorbent composite nonwoven web in which polymerization of superabsorbent is completely integrated into the process for forming the fibrous nonwoven web substrate. The process may include the step of forming a plurality of particles, fibers or fibrils from the reacting superabsorbent polymer mixture of precursor materials. The process will then further include the step of providing at least one primary gas stream to contain the polymerizing superabsorbent material and carry it away from the extrusion or combining apparatus. An additional gas stream may be provided to contain the thermoplastic fibers (if provided) and carry them away from the extrusion apparatus. This additional gas stream may also, contain and carry at least one superabsorbent polymer precursor composition while a similar gas stream may carry an ingredient to cause the superabsorbent polymer precursor composition to begin polymerizing. The process further includes the steps of providing at least one secondary gas stream carrying a plurality of hydrophillic (for example, cellulose) fibers, merging the primary and secondary streams together to form an integrated stream containing a mixture of ingredients from the primary and secondary streams, and directing the integrated stream onto a forming surface to form the composite material.
One or more streams of superabsorbent polymer precursor composition are added to the cellulose fibers, staple fibers and/or thermoplastic fibers, desirably before the fibers contact the forming surface. This can be accomplished by a) adding the one or more streams of superabsorbent polymer precursor to the primary stream (with or without thermoplastic fibers) before it combines with the secondary stream, b) adding the one or more streams of superabsorbent polymer precursor to the cellulose fibers in the secondary stream before it combines with the primary stream, c) adding a first stream of superabsorbent polymer precursor to the primary stream (with or without thermoplastic fibers), and adding a second stream of superabsorbent polymer precursor to the cellulose fibers in the secondary stream, before the two streams are combined, and/or d) adding one or more streams of superabsorbent polymer precursor to one or more tertiary gas streams which are combined into the integrated stream before the cellulose fibers (with or without thermoplastic fibers) contact the forming surface. Again, the thermoplastic fibers may be omitted and the primary stream may be used only to add the first stream of superabsorbent polymer precursor.
The one or more streams of superabsorbent polymer precursor may chemically react to form a superabsorbent polymer directly on the surfaces of the staple fibers, thermoplastic fibers and/or cellulose fibers, as the absorbent composite nonwoven web is being formed on the forming surface. Alternatively, the one or more streams of superabsorbent polymer precursor may partially or completely react to form a superabsorbent polymer before coming into contact with the forming surface. A further alternative is that the superabsorbent polymer precursor partially reacts so as to form particles, fibers or fibrils before coming into contact with other fibers (cellulose, staple, and/or thermoplastic) and finish polymerizing after in contact with these fibers and even after the nonwoven web is formed on the forming surface.
Other ingredients may also be added to the absorbent composite nonwoven web via the primary, secondary, tertiary and/or additional streams. These other ingredients may include, for instance, elastomeric fibers, less hydrophilic synthetic fibers (e.g., nylon and polyester), already formed superabsorbent particles or fibers, odor scavenging particles or fibers, bonding agents, wetting agents, and the like.
Additional surface crosslinking may also be performed on the absorbent material with in-situ polymerized superabsorbent once it is on the forming surface. Said surface crosslinking may enhance the absorbent properties of the material.